The present invention relates to high resistance transparent coatings, and specifically methods for formulating such coatings so as to increase an electrical resistance thereof, and useful articles including such increased resistance coatings.
High resistance transparent coatings have numerous applications. For example, when copying images using electrostatic imaging, it is common to subject a film having a dielectric material on a surface thereof to an electrical potential so as to selectively dispose electrostatic charge thereon which is then developed by applying a toner onto the surface of the film.
The film itself comprises the dielectric material layered over a high resistance transparent coating which is laid over a substrate, such as polyester. In this case, the high resistance transparent coating functions as a ground plane and is therefore required to have an electrical resistance within a predetermined range such that the coating can function as required.
Specifically, if the electrical resistance of the ground plane coating is too low, say for example below 100,000 ohms per square, the ground plane coating is unduly conductive which results in ghost images being formed by electrodes in close proximity to other electrodes which create an electrical potential for depositing an electric charge on the dielectric. Alternatively, if the electrical resistance of the ground plane coating is unduly large, the time constant for deposition of charge on the film becomes too long resulting in very slow image formation, typical excessively high resistances being those above 20 meg ohms per square.
Furthermore, even though a range of electrical resistances for the ground plane coating exists for an electrostatic imaging process to work, oftentimes there are optimum resistance values which are desired to optimally create electrostatic images, depending on the exact type of electrostatic imaging process being used. A disadvantage in the art is that material compositions which yield a particularly desired optimum electrical resistance are generally unstable. Specifically, small variations in the material composition during manufacture tend to unduly vary the electrical resistance. Thereafter, in use changes induced in the composition due to environmental effects, such as oxidation migration for example, further change the electrical resistance.